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On May 1996, from orbit 25195 to 25488, all the ERS-1 SAR (RAW, SLC, PRI, GEC) data has been affected by a timing error significantly impacting the data georeferencing. As illustrated in fig. 1 the affected dataset presents a georeferencing discrepancy of ~6km in azimuth that avoids the data exploitation for application requiring consistent geolocation information like interferometry.

Although these orbits represent a small portion of the ERS-1 archive, they also represent a significant portion of the ERS tandem mission. However, it should be noted that the data is of nominal radiometric quality and as such could be used for applications where the geolocation information is not required.

Shortly after detection of the problem, palliative actions have been immediately undertaken. The affected datasets have been flagged in the ESA catalogues and the customers who ask for them are notified of the georeferencing discrepancy.

Despite these actions, the timing error still remains avoiding the complete exploitation the ERS tandem mission by the SAR/InSAR community. Therefore it has been recently decided to investigate further the anomaly. The nature of the ERS-1 timing error has been found and a solution to fully correct the problem has been proposed. Fig. 2 shows the same image as fig. 1 corrected from the timing error. Further details could be found on the e1_may96_timing_error.pdf document that presents the results of the investigation and the technical approach used.

>>e1_may96_timing_error.pdf

With the actual ESA standard ERS SAR processor, the VMP (Verification Mode Processor), the problem is still not handled. However with the close future the new ESA ERS processor PF-ERS, the time correlation problem will be fully fixed.

Download the entire set of corrected PATN files hereafter where the UTT/SBT time correlation is cosistent.

ERS-1 image acquired during May 1996 over the gulf of Naples close to mount Vesuvius. On both figures the coastlines are overlayed to illustrate the precision of the georeferencing.
fig. 1: Image processed with the wrong timing parameters. A discrepancy of ~6km between the image and the coastline layer is clearly visible. (Click to enlarge) fig. 2: Image processed with the good timing parameters. The coastline layer matchs with the image content. (Click to enlarge)